1. Field of the Invention
The present invention relates generally to a radio receiver and more particularly is directed to a receiver capable of demodulating either an AM stereophonic broadcast signal or a monaural broadcast signal.
2. Description of the Prior Art
It is known to provide a stereophonic broadcast in which only one AM broadcast wave is employed, for example, as disclosed in U.S. Pat. No. 4,194,088 in which a sum signal L+R of a left channel signal L and a right channel signal R is used to AM-modulate a carrier signal and a difference signal L-R is employed to phase-modulate the carrier signal. Further, the carrier signal is phase-modulated by a pilot signal Sp which indicates that broadcast is stereophonic. In such case, an AM broadcast wave Sb is expressed as follows: EQU Sb=(1+L+R)cos{ .omega..sub.c .multidot.t+.alpha.(L-R)+Sp}
where
.omega..sub.c is the carrier frequency (angular frequency); PA1 Sp=.beta.cos .omega..sub.p .multidot.t PA1 .omega..sub.p =2.multidot..pi..multidot.fp PA1 fp is the pilot frequency (5 to 100 Hz); and PA1 .alpha. and .beta. are each a constant (modulation degree).
A prior art receiver which will receive an AM stereophonic broadcast signal according to such system, may, for example, have a demodulating circuit which includes an AM detector receiving the usual IF signal to derive therefrom a monaural signal when receiving a monaural broadcast signal and to derive therefrom the sum signal L+R when receiving a stereophonic broadcast signal, a matrix circuit having a first input receiving the output of the AM detector, and a sub-channel demodulator also receiving the IF signal and deriving therefrom the difference signal L-R and the pilot signal Sp when an AM stereophonic broadcast signal is being received, with such difference signal L-R being applied to a second input of the matrix circuit so that the latter will provide left and right stereophonic signals at respective outputs when an AM stereophonic broadcast signal is received. A mode selecting switch is interposed between the sub-channel demodulator and matrix circuit, and such switch is closed to establish the stereophonic mode of the receiver, in which case, the difference signal L-R is transmitted by the switch to the matrix circuit, or the switch may be opened for establishing the monaural mode of the receiver. Further, in the known receiver, the pilot signal appearing in the output of the sub-channel demodulator when a stereophonic broadcast signal is received is extracted from such output by a band-pass filter and is processed to provide a corresponding voltage which is compared with a reference voltage in a level comparator to provide a corresponding compared output for controlling the mode selecting switch. Generally, when the extracted pilot signal appears in the output of the band-pass filter, the corresponding voltage is at a level relative to the reference voltage to cause the compared output to close the switch and thereby establish the stereophonic mode. On the other hand, when a monaural broadcast signal is being received, so that no pilot signal can be extracted from the output of the sub-channel demodulator, corresponding voltage is at such a level relative to the reference voltage as to cause the compared output to open the switch for establishing the monaural mode of the receiver.
However, when the above described prior art receiver is being tuned or detuned, the frequency or phase of the IF signal is changed and the demodulating circuit cannot distinguish the frequency or phase change of the IF signal due to tuning and detuning from the pilot signal which is intended to identify the reception of a stereophonic broadcast signal. In other words, a disturbance signal caused by the frequency or phase change of the IF signal upon tuning or detuning or by external noise is of a frequency similar to that of the pilot signal so that it also appears in the output of the sub-channel demodulator and may be extracted therefrom by the bandpass filter intended to extract the pilot signal. Therefore, even in the absence of an extracted pilot signal due to the fact that a monaural broadcast signal is being received, the disturbance signal occurring in response to tuning or detuning, or in response to external noise, may be erroneously identified as a pilot signal to cause erroneous establishment of the stereophonic mode while a monaural broadcast signal is being received.